Apparatus and method for interfacing time-variant signals

ABSTRACT

An improved apparatus and method for interfacing a time variant waveform between two hardware environments. In one aspect, the invention comprises a circuit for accurately simulating the output of one or more types of sensing device (e.g., passive bridge pressure transducer) for use with a plurality of different monitoring and/or analysis devices, thereby obviating the need for specialized interface circuitry adapted to each different monitor/analyzer. In one exemplary embodiment, the sensing device comprises a non-invasive blood pressure monitor (NIBPM), which universally interfaces with prior art patient monitors via the interface circuit of the invention. In a second aspect of the invention, an improved NIBPM device incorporating the interface circuit is disclosed. An improved disconnect circuit adapted to sense the status of the electrical connection between the sensing device and monitor is also described.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to apparatus and methods forinterfacing time-variant signals, and specifically to theconditioning/simulation of waveforms (such as, for example, bloodpressure waveforms) on monitoring equipment.

2. Description of Related Technology Slowly time-variant waveforms orsignals have broad application in many fields, including industry,research, and medicine. Within the medical field, one application ofparticular interest relates to the monitoring of arterial blood pressurewithin living subjects (such as human beings and canines). As part ofso-called “invasive” arterial blood pressure monitoring, a disposablepressure transducer is commonly utilized to measure the blood pressureof the subject via an invasive arterial line that infuses normal salineat a “KVO” rate into an appropriate blood vessel. KVO (or “keep veinopen”) refers to the minimum flow rate required to keep an IV needlefrom clotting off in the vein. The disposable pressure transducer (DPT)is a passive resistive bridge implemented typically from silicon strainbeam technology. The DPT electrically interfaces to a patient monitoravailable from any number of different vendors such as HewlettPackard/Agilent/Philips, General Electric/Marquette, Datex-Ohmeda,Datascope/Fakuda-Denshi, Welch Allyn/Protocol, Space Labs, Criticare,Critikon, and Valley Medical, as well as others. The patient monitorsupplies the excitation signal to energize the bridge circuit of theDPT, and also provides the signal conditioning of the output derivedfrom the DPT in order to display the subject's blood pressure waveformon the display device of the patient monitor.

The various monitors described above utilize varying methods to energizethe bridge and recover the output signal. In one simple scheme (shown asFIG. 1), the bridge 101 of the DPT 100 is driven by a constant directcurrent (DC) voltage source 102. The value of the DC voltage produced bythe source 102 can be any value between 1V and 10V, but is typically setat 5V. This voltage value minimizes the self-heating effect that occurswhen resistors R1 104, R2 106, R3 108, R4 110, R5 112, and R6 114dissipate power. This power dissipation changes the temperature of thebridge sensor resistors R3 108, R4 110, R5 112, and R6 114, and causestheir electrical resistance values to change. This change in resistancecauses an error in the output of the bridge 101 which is not desired.Usually the manufacturer of the DPT will add additional resistors (notshown) to compensate for such temperature effects, and to calibrate theoutput of the DPT to a particular value (such as 5 uV/V/mnHg).None-the-less, minimizing the self-heating of the components limits themagnitude of the error, since such electrical compensation is neverperfect.

As shown in FIG. 1, the drive signal to the DPT 100 is applied acrossthe +E 120 and −E 122 terminals. Resistors R1 104 and R2 106 limitbridge current, and provide temperature compensation for span. Whenpressure is applied to the transducer, the geometry of the siliconstrain beam is arranged so that resistors R3 108 and R6 114 decrease invalue, and the values of resistors R4 110 and R5 112 increase in valueby the same amount. This change is typically on the order of 1% of theresistor's resistance value, and is commonly referred to as “Delta R” orΔR. In a typical configuration, resistors R3 108, R4 110, R5 112, and R6114 all have the same nominal value referred to as R_(b). Resistors R1and R2 are usually chosen to have equal resistance so that the nominalvoltage at terminals +S 130 and −S 132 will be equal to one-half of thedrive voltage E_(d). For these conditions, the output impedance of thebridge 101 will be R_(b), and the input impedance will be given by Eqn.1:Zin=R1+R2+Rb  (Eqn. 1)Typical values for Zin and Zout (R_(b)) are 3 K ohms and 300 ohms,respectively. Given these definitions, it can be readily demonstratedthat the bridge output voltage across terminals $\begin{matrix}{{Es} = {{{+ S} - \left( {- S} \right)} = {{Ed} \times \frac{dR}{Zin}}}} & \left( {{Eqn}.\quad 2} \right)\end{matrix}$Derivation of Eqn. 2 is conducted by analyzing an equivalent balancedbridge configuration 200 (FIG. 2) based on the following sixassumptions: (i) Resistor R1 202=R2 202; (ii) Resistors R3 204, R4 206,R5 208, and R6 210 are all equal in resistance when the bridge 200 is inbalance, and are equal to R_(b); (iii) when the bridge unbalances,resistors R3, R4, R5, and R6 all change by an equal amount (dR) as shownin FIG. 3; (iv) the load across terminals +S 230 and −S 232 is aninfinite differential impedance; (v) there are no loads between +S and+E or −E; and (vi) there are no loads between −S and +E or −E.

When the bridge in FIG. 2 is in balance, the resistance between terminal1 240 and terminal 2 242 is given by R_(b) (i.e., 2×Rb placed inparallel with 2×Rb yields an effective impedance of R_(b)). In theunbalanced configuration of FIG. 3, the resistance between terminal 1340 and terminal 2 342 is also given by R_(b), since the quantity(R3+R4+dR−dR)=2R_(b), which, when placed in parallel with(R5+R6+dR−dR)=2R_(b), yields an effective impedance of R_(b). Similarly,the bridge output impedance Zout=R_(b).

The unbalanced circuit of FIG. 3 can be simplified to an equivalentcircuit 400 shown as FIG. 4 herein. The input impedance seen by theexcitation source E_(d) is therefore given by Eqn. 3:Z _(in) =R1+R2+Rb  (Eqn. 3)The relationship between Eb (i.e., the equivalent voltage across thebridge) and the excitation voltage E_(d) is given by Eqn. 4:$\begin{matrix}{{Eb} = {{Ed} \times \left\lbrack \frac{Rb}{{R\quad 1} + {R\quad 2} + {Rb}} \right\rbrack}} & \left( {{Eqn}.\quad 4} \right)\end{matrix}$Note, however, that (R1+R2+Rb)=Z_(in). Hence, Eb can be represented asshown in Eqn. 5: $\begin{matrix}{{Eb} = {{Ed} \times \left\lbrack \frac{Rb}{Zin} \right\rbrack}} & \left( {{Eqn}.\quad 5} \right)\end{matrix}$The bridge may be analyzed alone, since the voltage E_(b) across thebridge is constant (and hence R_(b) is constant). This un-balancedcircuit equivalent 500 is shown in FIG. 5 herein. The voltage at node +S502 is given by Eqn. 6: $\begin{matrix}{{E\left( {+ S} \right)} = {{{Eb} \times \left\lbrack \frac{{Rb} + {dR}}{{Rb} + {dR} + {Rb} - {dR}} \right\rbrack} = {{E\left( {+ S} \right)} = {{{Eb} \times \left\lbrack \frac{{Rb} + {dR}}{2 \times {Rb}} \right\rbrack} = {{E\left( {+ S} \right)} = {{Eb} \times {\left\lbrack {\frac{1}{2} + \frac{dR}{2 \times {Rb}}} \right\rbrack.}}}}}}} & \left( {{Eqn}.\quad 6} \right)\end{matrix}$A similar expression can be developed for the voltage at node −S 504(Eqn. 7): $\begin{matrix}{{E\left( {- S} \right)} = {{Eb} \times \left\lbrack {\frac{1}{2} - \frac{dR}{2 \times {Rb}}} \right\rbrack}} & \left( {{Eqn}.\quad 7} \right)\end{matrix}$The differential output E(s) is simply E(+s)−E(−s), as follows:$\begin{matrix}{{E(S)} = {{{Eb} \times \left\{ {\left\lbrack {\frac{1}{2} + \frac{dR}{2 \times {Rb}}} \right\rbrack - \left\lbrack {\frac{1}{2} - \frac{dR}{2 \times {Rb}}} \right\rbrack} \right\}} = {{E(S)} = {{Eb} \times \left\lbrack \frac{dR}{Rb} \right\rbrack}}}} & \left( {{Eqn}.\quad 8} \right)\end{matrix}$Finally, substituting the result of Eqn. 5 into Eqn. 8 yields Eqn. 2above:${E(s)} = {{Ed} \times \left\lbrack \frac{dR}{Zin} \right\rbrack}$

In order for all DPT's to function similarly, they are typicallycalibrated during manufacture to a standard sensitivity of 5 uV/V/mmHg.This means that for an applied pressure of 100 mmHg, and a drive voltage(E_(d)) of 5V, the output E_(s) will be 2.5 mV. This requires that theresistance difference (dR) be 1.5 ohms, according to Eqn. 2 above. For aZin of 3 kohms, the full-scale pressure specification for such DPTs is300 mmHg, which would require a resistance difference (dR) value of 4.5ohms, and yield an output voltage of 7.5 mV.

It can be shown that the differential resistance dR is a function ofpressure:dR=Ks×Zin×P  (Eqn. 9)where:

K_(s)=scaling factor of 5 uV/V/mmHg, and

P=sensed pressure.

Substituting this result into Eqn. 2 results in the transfer functionfor the bridge, Eqn. 10:Es=Ed×Ks×P  (Eqn. 10)Note that the output voltage Es is a function of both pressure (P), theinput variable, and the drive voltage (Ed) provided by the monitor.

For the 5 VDC drive condition, the signal processing of the output istypically quite limited in scope; e.g., amplifying the bridge outputwith an instrumentation amplifier, and filtering the output with a2-pole low pass filter whose cutoff frequency is above any frequencycomponents of interest in the blood pressure signal. A typical valve forsuch filter cutoff frequency is 45 Hz.

When a user wishes to supply the monitor with and display a waveformother than that derived from the aforementioned DPT (such as that from adigital non-invasive blood pressure monitor such as that manufactured bythe Assignee hereof), the DPT must be disconnected from the monitor, andthe new signal source electrically substituted. For the case of a fixedDC drive voltage of 5V, a circuit may be readily fashioned to interfacethe new (e.g. digital) signal source to the patient monitor, such asthat shown in FIG. 6.

As illustrated in FIG. 6, resistor R1 602 of the interface circuit 600allows the patient monitor detect the 3 Kohm impedance value (Zin) itnormally sees when using the DPT. Additionally, resistors R2 606 and R3608 set the differential output impedance (Z_(out)) to 300 ohms. Theoutput of node S+610 is biased to +2.5 V by reference amplifier U1 616,and amplifiers U2 618 and U3 620 (and their associated components) setthe −S output according to Eqn. 11:−S=2.506−0.0025×Ein  (Eqn. 11)The differential output between +S and −S is given by Eqn. 12:Es=−0.006+0.0025×Ein  (Eqn. 12)The fixed offset of −6 mV (for the exemplary circuit of FIG. 6) can be“zeroed out” or cancelled by most patient monitors, leaving the outputvoltage E_(s) a function of the input voltage, and scaled such that a 1Vinput equals 100 mmHg. An alternative to nulling out the −6 mV with themonitor is to add a zero adjustment to amplifier U2 618.

While the circuit 600 of FIG. 6 (or any other similar circuit) worksgenerally for any monitor that has a fixed +5V DC constant voltageexcitation, it has significant shortcomings when one attempts to drivethe many different types of patient monitors presently available. Manysuch patient monitors do not use constant +5V excitation, but rather usebipolar sine wave drive, or even pulse drive as a carrier between 2 KHzand 5 KHz, which is modulated by the pressure signal. These monitors usean AC drive to reduce bridge offset effects, and cancel noise.Furthermore, they require a synchronous demodulator as part of theirsignal conditioning circuitry, in order to recover the blood pressuremodulation signal.

Therefore, in order to drive these monitors with a non-DPT device suchas the aforementioned digital NIBPM, a circuit is needed that mimics theelectrical profile and operation of a passive transducer bridge. Thetransfer function of such circuit must be effectively identical to thatof the passive bridge, and the input and output impedances must matchthose of the passive bridge as well. The sensitivity factor of 5uV/V/mmHg previously described (or any corresponding value for theselected monitor) must also be maintained. The circuit must functionwith any type of excitation source including constant voltage drive ofeither polarity, constant current drive of either direction, and any ACvoltage or current drive of any waveshape, duty cycle, and DC offsetwith a frequency of 1 KHz to 10 KHz.

Based on the foregoing, what is needed is an improved apparatus andmethod for interfacing sensing devices producing time variant waveforms(such as for example the systolic, diastolic, and/or average bloodpressure waveforms of a living subject) with monitoring devices. Suchapparatus and method would ideally (i) be readily adapted to a varietyof different configurations of monitoring or display devices, (ii) havea wide dynamic range; (iii) be able to operate on binary digital input(versus requiring conversion to analog first); (iv) maintain the desiredsensitivity factor; (v) function with any type of excitation sourceincluding constant voltage drive of either polarity; (vi) function witha constant current drive of either direction, and (vii) function withany AC voltage or current drive of any waveshape, duty cycle, and DCoffset. Furthermore, such circuit would optimally be electricallyisolable from the monitor, be stable and have minimal error or driftthrough its simulation range, and include provision for the detectingwhen the monitoring device was electrically connected thereto.

SUMMARY OF THE INVENTION

The present invention satisfies the aforementioned needs by providingimproved apparatus and methods for interfacing time-variant signalsbetween different hardware environments, including the blood pressurewaveforms obtained from a living subject.

In a first aspect of the invention, an apparatus useful for interfacinga time-variant signal between two hardware environments is disclosed. Inone exemplary embodiment, the apparatus is adapted to accuratelysimulate one or more types of passive bridge pressure transducer. Theinterface apparatus is configured to simulate any signal that thepressure transducer could produce, including waveforms having frequencyranging from direct current (i.e., 0 Hz) up through several hundredHertz. In one variant of this apparatus, a circuit adapted to simulatethe one or more blood pressure waveforms as would be produced by adisposable passive bridge pressure transducer (DPT) is provided. Thecircuit comprises a digital-to-analog converter (DAC) receiving digitalinputs from the associated blood pressure sensing and processingapparatus (e.g., a pressure-based tonometric systems, or combinedpressure/Doppler-based ultrasonic system, and its attendant signalprocessing), which are then conditioned using a linear transfer functionwhich replicates that of a DPT bridge device. The circuit uses theexcitation signal from the patient monitor as the reference, andadvantageously requires no analog signal synthesis; accordingly, nodependency upon voltage references or errors inherent therein ispresent. The circuit is further “universal” in nature, being adapted tointerface with essentially any different configuration of monitoringdevice, regardless of the type of excitation used by that monitoringdevice for the DPT, or type of output signal conditioning applied. Thecircuit also comprises an adjustable scale factor, such factor beingbased on the adjustment of the ratio of two resistance values, andinherently has very low drift, thereby further enhancing accuracy.

In a second aspect of the invention, a method for simulating atime-variant output signal from a first device using a second device isdisclosed. The method generally comprises providing the second device;providing an excitation voltage to the second device; generating adigital representation of a time-variant waveform using the seconddevice; applying a transfer function to the digital representation, thetransfer function being substantially similar to that for the firstdevice; and generating an output signal based at least in part on thedigital representation and the transfer function, the output signalbeing substantially similar to that produced by the first device. In oneexemplary embodiment, the first device comprises a passive bridgeelement, the time-variant waveform comprises a blood pressure waveformobtained from a living subject, and the second device comprises anon-invasive blood pressure monitor (NIBPM). The NIBPM is connected to aconventional monitoring device typically used with prior art passivebridge DPT devices, the presence of the NIBPM being detected through thepresence of a specified impedance (or alternatively, a voltage at apredetermined terminal of the monitor). The method further comprisesbuffering the excitation signal provided to the NIBPM, and disposing theDAC in feedback loop arrangement whereby the internal current thereofflows is a function of a predetermined parameter (e.g., the DAC count,N).

In a third aspect of the invention, an improved disconnect circuit fordetecting when a monitoring device (such as a patient monitor) isdisconnected from its sensing apparatus (e.g., Doppler blood pressuresensor) is disclosed. The disconnect circuit detects the presence of amonitor by detecting a signal associated with the monitor; e.g., thedrive signal the monitor uses to excite the passive bridge normally usedwith the monitor. Rather than evaluating this signal directly, thedisconnect circuit of the present invention evaluates a buffered versionof the signal. In one exemplary embodiment, the disconnect circuit isphysically disposed in the sensing apparatus. A window comparatortransfer function is used for detection; thus, as long as some part ofthe drive signal waveform exceeds a predetermined signal magnitude, theoutput of the window comparator will be held in a predetermined state.This approach ensures that any signal of any shape or duty cycle will bedetected by the circuit. A comparatively long time constant is also usedto avoid zero-crossing waveforms or other momentary voltage dips frominducing unwanted or spurious artifacts.

In a fourth aspect of the invention, an improved apparatus fornon-invasively measuring the blood pressure of a living subject isdisclosed. In one exemplary embodiment, the apparatus comprises (i) anultrasonic Doppler-based system adapted to measure the hemodynamicproperties associated with the subject, such as blood kinetic energy orvelocity, and determine arterial blood pressure there from, and developat least one binary digital signal related thereto; and (ii) theinterface circuit previously described. The hemodynamic parametermeasurement system comprises a signal processor operatively coupled toan ultrasonic transducer and a pressure transducer, and an applanationdevice adapted to control applanation pressure applied to thetransducer(s). The signal processor (and associated algorithms) generatea calibration function and determine blood pressure based on themeasured data and the derived calibration function. The blood pressuremeasurement data is fed to the interface circuit, which conditions thesignal so as to allow data communication with a monitor. The interfacecircuit is adapted to communicate data with literally any type ofmonitor device (regardless of manufacturer), and hence the apparatus mayadvantageously be used to non-invasively measure blood pressure from asubject irrespective of the in situ monitoring equipment available. In asecond embodiment, the ultrasonic system measures various parametersassociated with the blood vessel of the subject, and determines bloodpressure based at least in part by calculating time frequencydistributions for the collected data. In yet another embodiment, theapparatus is configured with a wireless (e.g., radio frequency) datalink such that digital data representative of the subject's bloodpressure is transmitted to the interface circuit disposed proximate tothe patient monitor, thereby obviating the need for electrical cords.

In a fifth aspect of the invention, an improved apparatus for monitoringthe blood pressure of a living subject is disclosed. The apparatusgenerally comprises a non-invasive blood pressure monitoring (NIBPM)device coupled via the aforementioned simulation circuit to a monitorsystem, the latter adapted to monitor and optionally analyze, display,and record blood pressure waveform data (and other relevant data)relating to a given subject. In one exemplary embodiment, the NIBPMapparatus comprises an ultrasonic Doppler-based system adapted tomeasure the hemodynamic properties associated with the subject, such asblood kinetic energy or velocity, and determine arterial blood pressurethere from, and develop at least one binary digital signal relatedthereto. The interface circuit previously described herein is used toprovide hardware interface between the NIBPM and the monitor. Themonitor comprises a device adapted to receive the analog signal from theNIBPM via the interface circuit, and analyze the data within theprocessing of the monitor in order to derive (and display) the resultantmeasured value of blood pressure.

In a sixth aspect of the invention, an improved method of providingtreatment to a subject using the aforementioned apparatus and method isdisclosed. The method generally comprises: obtaining data from thesubject using a sensing device; generating a first signal based at leastin part on the obtained data; conditioning the first signal using aconditioning circuit to produce a second signal; providing the secondsignal to a monitoring device, the latter producing a representation ofa desired parameter, and providing treatment to the subject based on theparametric representation. In one exemplary embodiment, the dataobtained from the subject comprises arterial hemodynamic data (e.g.,pressure, velocity, kinetic energy) obtained from the radial artery ofthe human being, and the sensing device comprises the aforementionedultrasonic NIBPM system with associated interface circuit. The digitalrepresentation of arterial blood pressure generated by the NIBPM systemis input to the interface circuit, which conditions the signal for“universal” use by any monitoring system. The selected monitoring systemtakes the conditioned blood pressure signal and displays the waveform orother representation (such as digital values of mean, systolic, anddiastolic pressures) for use by the care-giver. The caregiver thenprescribes a course of treatment (such as the administrationpharmaceuticals, or additional monitoring) based on the displayedinformation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an exemplary prior art circuit used toenergize and recover the output signal from a passive resistance bridgeused in a disposable pressure transducer (DPT).

FIG. 2 is a schematic diagram of a balanced bridge circuit which iselectrically equivalent to that of FIG. 1.

FIG. 3 is a schematic diagram of the equivalent circuit of FIG. 2,except in an unbalanced condition.

FIG. 4 is a schematic diagram of an unbalanced bridge circuit which iselectrically equivalent to that of FIG. 3.

FIG. 5 is a schematic diagram of the unbalanced bridge circuit of FIG.4, excerpt wherein the circuit is simplified to reflect the bridgeresistance R_(b) alone.

FIG. 6 is a schematic diagram of an exemplary prior art circuit adaptedto interface a non-DPT signal source to a fixed DC drive voltage patientmonitor.

FIG. 7 is a schematic diagram of an exemplary prior art passive bridgedevice illustrating the response thereof to a sine wave excitationsignal applied at the drive input of the bridge.

FIG. 8 is a schematic diagram of one exemplary embodiment of theinterface circuit of the invention.

FIG. 9 is partial schematic of a second embodiment of the interfacecircuit of the invention, wherein optical isolators are disposed in thedata path to the DAC, thereby providing electrical isolation.

FIG. 10 is a schematic diagram of one exemplary embodiment of thedisconnect circuit optionally used in conjunction with the interfacecircuit of FIG. 8 or 9.

FIG. 10 a is a graph illustrating the operation of the windowingfunction associated with the comparator of the disconnect circuit ofFIG. 10.

FIG. 11 is a schematic diagram of a third embodiment of the interfacecircuit of the invention, also comprising self-test functionality.

FIG. 12 is a logical flow diagram illustrating one exemplary embodimentof the method of simulating the time-variant output of a first deviceusing a second device adapted according to the present invention.

FIG. 13 is a functional block diagram of one exemplary embodiment of theapparatus for non-invasively measuring the blood pressure of a livingsubject according to the invention.

FIG. 13 a is a functional block diagram of a second embodiment of theapparatus for non-invasively measuring the blood pressure of a livingsubject, wherein the interface circuitry is disposed proximate to thepatient monitor.

FIG. 14 is a functional block diagram of a third embodiment of theapparatus for non-invasively measuring the blood pressure of a livingsubject, incorporating a wireless (e.g., radio frequency ISM band) datalink.

FIG. 14 a is a perspective view of one embodiment of the monitorreceiver unit of the apparatus of FIG. 14, illustrating the form factorthereof.

FIG. 15 is a logical flow diagram illustrating one exemplary embodimentof the method of providing treatment to a subject using theaforementioned apparatus and methods.

DETAILED DESCRIPTION OF THE INVENTION

Reference is now made to the drawings wherein like numerals refer tolike parts throughout.

It is noted that while the invention is described herein primarily interms of a method and apparatus for assessing the hemodynamic parametersof the circulatory system via the radial artery (i.e., wrist) of a humansubject, the invention may also be embodied or adapted to monitor suchparameters at other locations on the human body, as well as monitoringthese parameters on other warm-blooded species. Furthermore, in thebroader sense, the invention may be readily applied outside the medicalfield, such as for example to pressure monitoring devices in fluidicsystems, where it is desirable to have universal functionality orinterconnection between the sensing device and an associated monitoringdevice. It can be used to drive the signal processing circuits for anybridge-type transducer monitor that processes time-variant signals(e.g., 200 Hz or less). All such applications, adaptations and alternateembodiments are considered to fall within the scope of the claimsappended hereto.

As used herein, the term “non-invasive blood pressure monitor” or“NIBPM” refers to any apparatus adapted to measure or estimate the bloodpressure within a blood vessel of a living subject in either partiallyor completely non-invasive manner, regardless of method used.

Referring now to FIG. 7, the response of an exemplary prior art passivebridge device to a sine wave excitation applied at the drive input ofthe bridge is analyzed. Assuming perfect matching in the bridge 700, theoutputs +S 702 and −S 704 are also sine waves in phase with theexcitation voltage, but at one-half the amplitude of the drive voltage.If the bridge 700 senses an applied pressure (for example, of 100 mmHg),its resistance is altered by an amount equal to delta R, and the outputsine wave amplitudes at +S 702 and −S 704 change by 1.25 mV each, but indifferent directions. The differential output of the exemplary bridge700 of FIG. 7 will be a sine wave in phase with the drive, with anamplitude of 2.5 mV. The amplitude of this differential signal willfollow the blood pressure waveform applied to the device, which variesat a much lower frequency. Specifically, the period of the bloodpressure signal can range between about 1.7 sec (35 beats/minute) to0.25 sec (240 beats/minute). The blood pressure waveform can be obtainedby synchronously demodulating the output signal with the driveexcitation sine wave.

Recalling the transfer function of the passive bridge, Es=Ed×Ks×P (Eqn.10 above), it can be seen that the output of any interface circuit usedto universally mimic or “simulate” the passive bridge must be a functionof both the pressure signal P, and also a function of the driveexcitation source E_(d). Further, the transfer function shows that amultiplication must take place between the variables for the interfacecircuit to function the same as the passive bridge. Since Ed can bebipolar, at least 2 quadrant multiplication is implied.

Since the interface circuit is required to work for both DC drive, andAC drive, good DC stability is required. Since the interface circuit isscaled to look like a transducer, its output is 25 uV/mmHg with a 5 VDCdrive. If the drift error must be less than 1 mmHg over the operationaltemperature range of 10° C. to 40° C., the drift must be less than 0.833uV/° C. Furthermore, DPT's are only allowed a DC offset of 1.875 mV (75mmHg) by specification, so the interface circuit must meet thatrequirement as well.

Referring now to FIG. 8, a first embodiment of the improved interfacecircuit 800 of the invention meeting the foregoing requirements isdescribed. A shown in FIG. 8, the circuit 800 generally acts as aninterface between the non-invasive blood pressure measurement system(such as that manufactured by the Assignee hereof) and prior artmonitoring devices manufactured by any number of different entitieswhich are adapted for passive bridge transducer or similar sensingdevices (not shown). Specifically, jack or connector J1 802 connects tothe monitor 806 which supplies drive excitation voltage E_(d) at pin 1(+E) 808. Resistors R1 810 and R4 812 provide the input impedance,nominally selected in the present embodiment to be 3 K ohms. Duringoperation with the aforementioned passive bridge device, the monitor 806looks for such an impedance in order to determine if the passive bridgetransducer is electrically connected. Alternatively, the monitor maydetect the presence of a passive bridge transducer by looking for thesignal at +S 814 or −S 816.

In the embodiment of FIG. 8, resistors R1 and R4 form a 2:1 divider andapply a signal of E_(d)/2 to the inputs of two operational amplifiersU1B 817 and U2A 818. The operational amplifiers 817, 818 each compriseModel No. OPA2277 integrated circuit (IC) amplifiers manufactured byTexas Instruments Corporation, although other types, integrated orotherwise, may be used. The construction and operation of suchoperational amplifiers are well known in the electronic arts, andaccordingly are not described further herein. Both operationalamplifiers are configured as voltage followers (assume no current flowin resistor R5 820 for purposes of the instant discussion), so that thevoltage of their output signals are also represented by the relationshipE_(d)/2. Resistors R2 822 and R3 824 couple the output signals of theamplifiers to the +S 814 and −S 816 terminals of the transducer outputconnector J1 802, and provide a differential output impedance of 300ohms to the monitor, although it will be appreciated that otherresistance values may be substituted. The aforementioned OPA2277amplifier is an extremely low offset (e.g., 25 uV) and low drift (e.g.,0.1 uV/° C.) device, so the output offset is only on the order of 50 uVworst case (which corresponds to approximately 2 mmHg pressure). Totaldrift of this stage is only on the order of 3 uV over a 10° C. to 40° C.temperature range.

As shown in the exemplary embodiment of FIG. 8, the excitation voltageE_(d) is buffered by unity gain amplifier UIA 826 of the type well knownin the art, and applied to the reference input of a 12-bit, bipolar,multiplying digital-to-analog converter (DAC) U3 830. While the presentembodiment uses a 12-bit DAC, it will be recognized that DACs havingother bit resolutions (such as 8-bit, 10-bit, 14-bit, or 16-bit) andoperating characteristics (e.g., “flash” DACs) may also be utilized.Note also that in the present embodiment, the DAC supports the bipolarmultiplication function. The digital inputs 832, 834, 836 of the DAC 830carry the blood pressure waveform signal of the present embodiment.Among other functions, the DAC 830 performs the required multiplicationfunction described by the bridge transfer function, namely that of Eqn.13Ed×P  (Eqn. 13)The DAC 830 is connected in a feedback loop 840 with operationalamplifiers U2B 842 and U4 844 to form a bipolar, programmable, floatingcurrent source. In operation, the voltage at the reference input 848 ofthe DAC 830 causes an internal current to flow, the magnitude of whichthat is a function of the DAC count, N. This current flows through aninternal feedback resistor (10 K ohms in the present embodiment)connected to pin “2” 850 of the DAC 830. The DAC summing junction (notshown) is held at 0 volts by the feedback action of amplifier U2B 842.The output voltage at pin “2” of the DAC 830 is given by Equation 14:$\begin{matrix}{{PDAC} = {{- {Ed}} \times \frac{N}{4096}}} & \left( {{Eqn}.\quad 14} \right)\end{matrix}$This signal is supplied by instrumentation amplifier U4 844, which isconfigured for a gain of 1 (unity) in the present embodiment. By virtueof its connection, the voltage across resistor R6 854 becomes:$\begin{matrix}{{{ER}\quad 6} = {{Ed} \times \frac{N}{4096}}} & \left( {{Eqn}.\quad 15} \right)\end{matrix}$Amplifier U2B 842 will adjust its output voltage to satisfy Eqn. 15above. The voltage at the other end of resistor R6 854 is equal toE_(d)/2. Thus, the voltage at the output of amplifier U2B 842 becomes:$\begin{matrix}{{E\left( {U\quad 2B\quad 7} \right)} = {\frac{Ed}{2} - \left\lbrack {{Ed} \times \frac{N}{4096}} \right\rbrack}} & \left( {{Eqn}.\quad 16} \right)\end{matrix}$The output current which flows through R6 854, also flows through R5820, which adds to the signal at the output of U2A 818. It can be shownthrough analysis that the current flowing through R6 854 is given by thefollowing relationship: $\begin{matrix}{I = {\frac{E\quad d}{R\quad 6} \times \frac{N}{4096}}} & \left( {{Eqn}.\quad 17} \right)\end{matrix}$Where N is the count programmed into the DAC 830 via theserial-to-parallel interface (SPI) (pins “5”, “6”, and “7”) 832, 834,836. In the illustrated embodiment, N can be set for any value from 0 to4095. Hence, if E_(d) is 5.0 V and N=3000 counts, the current I will be44.388 uA per Eqn. 17. Note that as the drive voltage E_(d) reversespolarity, then the current I reverses direction, and the output signalat +S 814 reverses phase.

The current I develops a voltage across R5 820 given by the followingequation: $\begin{matrix}{{E\quad{R5}} = {R\quad 5 \times \frac{E\quad d}{R\quad 6} \times \frac{N}{4096}}} & \left( {{Eqn}.\quad 18} \right)\end{matrix}$This makes the output at U2A 818 equal to: $\begin{matrix}{{E\left( {+ S} \right)} = {\frac{E\quad d}{2} + \left\lbrack {R\quad 5 \times \frac{E\quad d}{R\quad 6} \times \frac{N}{4096}} \right\rbrack}} & \left( {{Eqn}.\quad 19} \right)\end{matrix}$Since the voltage at −S 816 is also E_(d)/2, then the differentialoutput is: $\begin{matrix}\begin{matrix}{{E\quad s} = \frac{K\quad s \times E\quad d \times N}{4096}} \\{{where}\text{:}} \\{{{K\quad s} = \frac{R\quad 5}{R\quad 6}},{{{and}\quad 0} \leq N \leq 4095.}}\end{matrix} & \left( {E\quad q\quad{n.\quad 20}} \right)\end{matrix}$Note that if the pressure P is substituted for the quantity N/4096, Eqn.20 has the same form as the transfer function for the passive bridgepreviously described herein.

The ratio of R5 820 to R6 854 sets the sensitivity value for thecircuit, and the scaling for the pressure transfer function. Forexample, for 5 uV/V/mmHg, Ks=0.002048. Note also that in the presentembodiment, the constants are arranged so that the value of N scales as0.1 mmHg/count. Thus N=3000 yields 3000×0.1=300 mmHg.

Additionally, in the present implementation, resistors R1 810 and R4 812are selected to be low tempco (25 ppm/° C.) and matched to 0.1%.Resistors R5 820 and R6 854 are also low tempco types (10 ppm/° C.), andhave accuracy better than or equal to 0.05%. Ideally, the resistors (R5AND R6) should be a custom network in a common thermal environment sothat differential thermal gradients are minimized. For the values of R5820 and R6 854 shown, the nominal accuracy of the output signal isadvantageously within 0.02% at 300 mmHg.

One of the advantages of the circuit 800 of the embodiment of FIG. 8 isthat the circuit is capable of utilizing a digital representation of theblood pressure (BP) input signal, as opposed to requiring an analogrepresentation of the BP signal. This capability is significant from theperspective that any deleterious effects on accuracy, drift, andnon-linearity associated with the analog synthesis are avoided, since noconversion between the digital and analog domains is performed prior toinput.

The circuit 800 is also advantageously ratio-metric with respect to thedrive signal E_(d) supplied by the monitor to which the circuit isconnected. Specifically, any change in Ed is also reflected in theproper ratio at the outputs +S and −S. Also, the output scaling of thecircuit 800 is adjustable, and the transfer function is inherentlylinear as described by equation 20, where Ks is the ratio of R5 to R6.As yet another advantage, digital processing of the (digital) BP signalis readily accomplished prior to inputting the signal to the DAC 830, soany BP signal anomalies and/or artifacts can be conveniently eliminatedthrough signal processing techniques of the type well known in the art.

Since blood pressure is a dynamic (i.e., time variant) signal, it isnecessary to send out a new digital representation of the BP value tothe DAC 830 whenever the BP value changes. The Assignee hereof has notedthat in an exemplary application, 1000 values per cardiac cycle providesufficient resolution, although other values (higher or lower) may beused consistent with the invention. In such application, each value of Ntakes approximately 3 usec to load, so updating of the DAC 830 can beperformed completely within 3 msec (i.e., 1000 values×3E-06 sec/value).In comparison, the fastest cardiac cycle occurs in roughly 250 msec. Inpractice, it can be shown that as few as 40 points per cardiac cycle issufficient to define typical blood pressure waveform signatures. Thisallows a sample rate as low as 160 Hz for the DAC in order toaccommodate cardiac cycles up to 240 beats per minute.

For the purposes of calibration, the DAC 830 can be set to N=0, and theattached monitoring device can be zeroed (via, for example, itsfront-panel transducer zero control). In the context of the previousexample, the DAC 830 can then be set to N=1000, which correlates to 100mmHg. This approach provides a constant output which is easily displayedon the monitor, and further provides a span calibration check. If forany reason the monitor displays a reading offset from the 100 mmHgcalibration value, then that value can be readily compensated for, suchas for example through insertion of an error term via the non-invasiveblood pressure monitor (NIBPM) with which the circuit is associated. Inone exemplary embodiment, the error term is entered through a keypadinput device associated with the NIBPM device manufactured by theAssignee hereof, although other methods (software or otherwise) may beused with equal success.

In another embodiment of the interface circuit 900 of the invention(FIG. 9), the digital programming lines 932, 934, 936 to the DAC 930 areoptically isolated via an optical isolator module 955, and an isolatedpower supply 957 of the type well known in the electronic arts used topower the circuit 900. This approach advantageously provides completefloating isolation of the interface circuit, and eliminates anypotential for ground loops, compromise of patient and/or operatorsafety, and the like.

It will be recognized that while the foregoing embodiments of theinterface circuit of the invention (i.e., FIGS. 8 and 9) are cast interms of discrete electrical components (i.e., operational amplifiers,resistors, capacitors, etc.) arranged on an exemplary printed circuitboard, the circuit, and even ancillary components associated therewith,may be embodied in one or more integrated circuit (IC) devices using anyof the well understood semiconductor design synthesis and fabricationtechniques known to those of ordinary skill. Such integrated circuit mayeven include the electronic components of the sensing device (e.g.,NIBPM) if desired. For example, the signal processor, ADC, and interfacecircuit 800 may all be embodied as an application specific integratedcircuit (ASIC) on a single silicon die. Other arrangements (includingthe use of multiple integrated circuit devices) are also contemplated,all such variants and alternate embodiments falling within the scope ofthe claims appended hereto.

Disconnect Circuit

Referring now to FIG. 10, one exemplary embodiment of the monitordisconnect circuit 1000 of the invention is described. This circuit 1000detects the presence of an electrically connected patient monitoringdevice by detecting the drive signal the monitor uses to excite thenormally connected passive bridge. Rather than detecting this signaldirectly, the circuit 1000 looks at a buffered version of the signal(Pin 1 of U1A 826 in FIG. 8), labeled E_(dbuf) 1010 in FIG. 10.

The construction and operation of the disconnect circuit 1000 is nowdescribed. As illustrated in FIG. 10, the drive signal Ed is applied tocomparators U8A 1012 and U8B 1014. The comparator threshold values areset by resistors R21 1016, R25 1018, and R27 1020 to a predeterminedvalue, +/−1V in the illustrated embodiment. The wired “or” connection1022 at the output of U8A 1012 and U8B 1014 forms a window comparatorarrangement 1024 and transfer function, as illustrated in FIG. 10 a.Thus, as long as some part of the drive signal waveform exceeds themagnitude of the predetermined value (e.g., |1V|), the output of thewindow comparator 1024 will be low. This configuration ensures that anysignal, whether DC or AC, and of any shape or duty cycle, will bedetected by the window comparator formed by the comparators 1012, 1014and output connection 1022. The low output impedance of the windowcomparator arrangement discharges capacitor C11 1026 through resistorR24 1028. As long as the voltage across C11 1026 is less than thethreshold of comparator U9A 1030 (e.g., +1V), the output of comparatorU9A 1030 will be low. If the drive signal E_(d) is lost due to anelectrically disconnected monitor, then the output of the windowcomparator arrangement 1024 goes to a high impedance state, and C11 1026starts to charge to a selected voltage (e.g., +5V) through resistor R221034. When the voltage across C11 1026 exceeds the predetermined value(e.g., +1V), comparator U9 1030 output goes high indicating that themonitor is electrically disconnected. The time constant (τ) for thisdetection is set by R22 1034 and C11 1026, and is approximately 100 mSfor the values shown in the illustrated embodiment. The use of acomparatively long time constant in the illustrated embodiment ensuresthat momentary loss of signal, such as periodic zero-crossings of a sinewave drive signal, are not detected by the circuit 1000. For theaforementioned 100 mS time constant, the drive signal frequency canadvantageously drop as low as about 100 Hz. In actual practice however,the drive signal frequency received by the circuit willcharacteristically remain much higher, typically on the order of 2 to 5KHz.

It is noted that the output from comparator U9A 1030 comprises a logiclevel-compatible signal that can be read by an input/output (I/O) portof any processor running from a 5V 1042 supply. Note that since U9A 1030is an open collector comparator, R23 1038 can be returned to any supplyvoltage such as 1.5V, 1.8V, or 3.3V for compatibility with a wide rangeof modern microprocessors or other parts.

Alternate Embodiment of Interface (and Disconnect) Circuit withSelf-Test

Referring now to FIG. 11, yet another embodiment of the interfacecircuit (with disconnect circuit) of the present invention is described.It will be noted that in the embodiment of FIG. 11, the circuit 1100 isadapted to provide enhanced functionality in various aspects as comparedto the embodiment of FIGS. 8 and 10, and FIG. 9, including self-testfunctionality, as described in greater detail below. The circuit 1100 ofFIG. 11 also has greater complexity, however, and accordingly may beoptimal for applications where its enhanced functionality is requiredand/or where the increased cost is not a significant issue.

As shown in FIG. 11, the supply voltages for the various analogcircuitry of the interface circuit 1100 are set to +/−7.5 in order toreduce the power requirements and heat generation rate of the circuit1100, and to increase its reliability. Additionally, the amplifier U11B1126 buffers the drive voltage Ed 1108 (+E) after the voltage dividerformed by R19 1110 and R17 1112, as compared to the arrangement of FIGS.8 and 10. The output of U11B 1126 is also configured to drive thereference input 1148 of the DAC U6 1130, and the comparators U1A 1170,and U1B 1172, via switch S1B 1174. The voltage Ed/2 1162 is also madeavailable to the A/D converter (not shown) for measurement. Furthermore,in order to compensate for the scale factor error that occurs due tothis configuration, the resistance values of resistors R7 1120, and R81154 have been changed, with the ratio of 244.1:1. These modificationskeep the circuit scale factor at 10 counts/mmHg.

Additionally, the reference voltages for the window comparator U1A 1170,and U1B 1172 of the circuit 1100 have been changed from +/−1 V to+/−0.5V. This is accomplished by changing the values of the resistors inthe divider; R3 1176, R4 1178, and R5 1180.

As shown in FIG. 11, the output comparator for the monitor detectorcircuit utilizes a Schmidt trigger logic gate, NC7WZ14 1181. Its outputcan drive either 3.3V or 5 V logic devices, although it will berecognized that other devices adapted to provide similar functionalityyet different dive voltages, may be substituted. The output of thecomparator 1181 is read directly by the microprocessor (or othercomparable processing device).

The circuit 1100 of FIG. 11 also includes radio frequency interference(RFI) protection via choke T1 1165, and capacitors C17 1166, C22 1167,C24 1168, and C28 1169. Electrostatic discharge (ESD) protection of thepatient monitor connector output 1102 is also provided via over-voltageclamp U9 1184. As a result of this protection, the range of the patientmonitor drive voltage (+E) 1108 is limited to +/−7.5V, or 15 V p-p.

As previously referenced, the circuit 1100 of FIG. 11 also includes aself-test sub-circuit 1177, which provides an independent means ofinsuring that the output signal to the patient monitor 806 is correct.This circuitry 1177 is now described in greater detail.

In order to have an independent means of insuring that the patientmonitor circuit is functioning correctly, the unknown external drivevoltage from the patient monitor (PM) 806 is disconnected, and a knownreference signal 1185 used instead. In the illustrated embodiment, theknown reference is generated by a precise voltage reference device (notshown), which provides +2.500 volts to S1A 1186. S1A 1186 and S1B 1174effectively form an electronic single pole double throw switch which iscontrolled by the state of control line TMAI 1187. In normal operationS1A is off, and S1B is on. When it is desired to check the function ofthe PM circuit, TMAI 1187 is asserted true, and S1A 1186 turns on andS1B 1174 turns off. This disconnects the external PM drive signal andconnects the known +2.5 V reference to the DAC U6, 1148 U7A 1117, U7B1118, U1A 1170, and U1B 1172. The MONITOR output 1182 should go high forthis condition, and can be verified by the microprocessor. This checksthe ability of the circuit to detect connection to the PM. To thecircuit, the +2.5 V reference behaves the same as a +5.000 V drivesignal from the external PM. The 2.5V reference 1185 can be read by asecond A/D converter (not shown) via the output of buffer U11A 1188 atREFMON 1189. This advantageously provides an independent check of thereference.

To verify the zero offset condition of the circuit 1100, the DAC 1130 isnow set to a count of 0000. Under this condition, which simulates a 0mmHg signal, the outputs of U7A 1117, and U7B 1118 are nominally +2.500volts. Ideally their difference would be zero, but both amplifiers havea finite offset of up to 25 uV in the illustrated embodiment. Thedifferential output can therefore be as much as 50 uV, which isequivalent to 2 nm hg for a 5 V drive. In order to measure this outputsignal U10 1190 is connected across the outputs of U7A 1117 and U7B 1118through resistors R13 1122 and R6 1124. U10's gain is set to 400 via R161191 and R15 1192. The output signal TLFDBK 1193 thus has a scale factorof 10 mV/mmHg. For the zero condition, TLFDBK 1193 will be in the rangeof +/−20 mV. The network of R14 1194, C23 1195, R12 1196, and C18 1197at the input of U10 1190, form a single pole low pass filter with anominal cutoff frequency of 1.6 KHz when configured as illustrated. Thisfilter prevents high frequency signals from entering U10 1190, andaffecting its output. U10 1190 itself has an offset which can be as muchas 50 uV. This offset must also be compensated since it represents +/−2mmHg as well. Switch S1C 1198 is used for this purpose. Switch S1C 1198is placed across the outputs U7A 1117, and U7B 1118 through resistorsR13 1122 and R6 1124. Switch S1C 1198 is turned on ZMAI 1199 is assertedtrue (logic 0). This shorts out the differential offset error of U7A1117, and U7B 1118 and allows the A/D converter (not shown) to measurethe offset error of U10 1190 at TLFDBK 1193. Switch S1C 1198 is thenopened, and the A/D (not shown) again measures the output of U10 1190 atTLFDBK 1193. This value represents the true “zero” signal beingpresented to the PM, and can become the baseline against which allnon-zero outputs are referenced.

It should be noted that by making differential measurements as in thepresent embodiment, all of the offset errors in the A/D measurementchannel advantageously cancel out. If either measurement falls outside apredetermined range, then a fault condition exists, and an alarm isgenerated (e.g., visual, audible, etc.), or other condition enabled(such the writing of data to a trace file). Note that the external PMmust be zeroed to compensate for the small differential output of +/−2mmHg. This is done when TMAI 1187 is 0, ZMAI 1199=1, and DAC 1130count=0000.

To verify the gain accuracy of the circuit 1100, the DAC 1130 is set toa count of 1000. This should result in an output signal to the PMrepresenting a predetermined value (e.g., 100 mmHg). Since U10 1190 isscaled at 10 mV/mmHg, its output signal TLFDBK 1193 should be 1.000 Vabove the zero reference value established earlier. If the measurementfalls outside a predetermined range, then a fault condition exists, andan alarm (or other condition) is generated. If both the zero offset andgain of the PM circuit are verified as OK, then the circuit is reset toits normal condition: TMAI 1187=0, and ZMAI 1199=1.

Another advantageous aspect of the self test sub-circuit 1177 is that itcan assess the proper operation of the monitor adapter interface circuit1100 during normal operation if the external PM uses a constant DC drivesignal. This is true for most modern monitors, although some still useeither a sine wave AC drive, or a pulsed DC drive. When a fixed DC driveis used, the A/D converter can measure the value of the drive signal atED/2 1162. By knowing this, the output for any pressure signal value canbe computed, and compared against the actual measured output at TLFDBK1193. If these values don't agree within a predetermined range, a faultcondition exists, and appropriate alarms would be generated (or otheraction taken). As an example of the foregoing, assume the externalpatient monitor drive is +6.000 Volts. The A/D would measure +3.000volts at ED/2 1162. Knowing that the actual drive voltage is2×+3.000=+6.000 volts, the output signal can be computed as in Eqn. 21below:E _(s)=(5 uV/V/mmHg)×(+6.000V)×400×P _(out),  (Eqn. 21)where P_(out) is expressed in mmHg. For a P_(out) value of 100 mmHg,(DAC 1130 count=1000) E_(s) will be equal to +1.200 V. E_(s) would bemeasured at TLFDBK 1193. Note that in the illustrated embodiment, theoutput signal must be referenced to a zero value DAC 1130=0000, in orderto cancel out the offset errors.

If the external PM 806 uses either a sine wave AC drive, or a pulsed DCdrive, then operation can only be verified by switching to the +2.500 Vreference. This can be done when the system first powers up, or whenmonitoring is suspended for some reason.

Method of Simulating Time Variant Output

Referring now to FIG. 12, the method for simulating a time-variantoutput signal from a first device using a second device according to theinvention is described. It will be recognized that while the followingdiscussion is cast in terms of a passive bridge transducer device suchas those previously described (“first device”) and its associatedmonitor, and a non-invasive blood pressure monitor (“second device”),the methodology of the invention may be readily applied to otherapplications and combinations of devices both within and outside themedical field, such as for example pressure measuring devices inindustrial fluidic systems.

As shown in FIG. 12, the first step 1202 of the method 1200 comprisesproviding the non-invasive blood pressure monitor (NIBPM); a tonometricpressure-based system such as that manufactured by the Assignee hereofis used in one embodiment, although myriad other types andconfigurations may be substituted. The NIBPM is then electricallyconnected to one of the aforementioned prior art patient monitoringsystems (e.g., those manufactured by General Electric, et al), per step1204. The connection of the NIBPM is optionally detected in step 1208through the presence of a specified impedance across two or moreterminals in the patient monitor (or alternatively, a voltage at apredetermined terminal of the patient monitor), based on an appliedexcitation voltage provided to the NIBPM by the patient monitor (step1206). In step 1210, the excitation signal is buffered within theinterface circuit 800, 900, 1100 as previously described. Note that anyof the foregoing interface circuits 800, 900, 1100 (or combinations ofthe desirable features thereof) may be used consistent with the method1200.

In step 1212, the NIBPM generates a digital representation of thetime-variant blood pressure waveform(s) through the data generated bypressure transducer elements (and optionally, acoustic transducerelements if so equipped) associated therewith. Generation of this signalis completely independent of the excitation signal applied by themonitor. The transfer function is next applied to the digitalrepresentation (step 1214) using the interface circuit 800, 900, 1100,the transfer function being substantially similar to that for thepassive bridge device as previously discussed.

Lastly, the output signal from the interface circuit 800, 900, 1100 isproduced which is compatible with the patient monitoring system (as wellas being “universally” compatible with the patient monitoring systems ofother vendors) per step 1216. The output signal of step 1216 is based atleast in part on the digital representation of the waveform (e.g., bloodpressure) provided to the DAC 830, 1130 of the circuit 800, 900, 1100,respectively, the applied drive signal 802, 1102 from the patientmonitor 806, and the applied transfer function, the output signal beingsubstantially similar to that produced by the passive bridge deviceunder comparable excitation signal and measured pressure waveform.

Apparatus for Hemodynamic Assessment

Referring now to FIG. 13, an improved apparatus for non-invasivelyassessing hemodynamic parameters (e.g., arterial blood pressure)associated with a living subject is described. In the exemplaryembodiment of FIG. 13, the apparatus 1300 comprises, inter alia, (i) ablood pressure measurement system 1302 adapted to measure the bloodpressure and/or other hemodynamic properties (such as for example bloodkinetic energy or velocity) associated with the subject, and develop atleast one binary digital signal related thereto; and (ii) the interfacecircuit 800, 900, 1100 previously described. The bloodpressure/hemodynamic parameter measurement system 1302 comprisesgenerally a signal processor 1304 operatively coupled to one or morepressure transducers 1308, as well as an applanation device 1310 adaptedto control applanation pressure applied to the transducer(s) 1308. Thesignal processor 1304 (and associated algorithms running thereon)determine arterial blood pressure based on the measured data. One keyadvantage of the present invention, in addition to its effectivelyuniversal adaptability to various types of monitors as previouslydescribed, is its adaptability to any number of different parametricmeasurement devices, whether adapted for medical applications such asblood pressure measurement or otherwise. For example, the presentinvention may be readily used with the apparatus and techniquesdescribed in detail in co-pending U.S. patent application Ser. No.09/534,900, entitled “Method And Apparatus For Assessing HemodynamicParameters Within The Circulatory System of A Living Subject” filed Mar.23, 2000, assigned to the Assignee hereof, and incorporated herein byreference in its entirety, may be utilized. As yet another alternative,the methods and apparatus of co-pending U.S. patent application Ser. No.09/815,080 filed Mar. 22, 2001 and entitled “Method And Apparatus ForAssessing Hemodynamic Parameters Within The Circulatory System Of ALiving Subject”, also assigned to the Assignee hereof and incorporatedherein by reference in its entirety, may be used with the presentinvention. It will be recognized, however, that yet even othertechniques for measuring blood pressure may be employed by the bloodpressure/hemodynamic assessment apparatus 1300 of the invention,including for example the time-frequency distribution based systemdisclosed in co-pending U.S. patent application Ser. No. 09/342,549,entitled “Method and Apparatus for the Noninvasive Determination ofArterial Blood Pressure” filed Jun. 29, 1999, or co-pending U.S. patentapplication Ser. No. 09/489,160, entitled “Method and Apparatus for theNoninvasive Determination of Arterial Blood Pressure” filed Jan. 21,2000, both assigned to the Assignee hereof, and both incorporated hereinby reference in their entirety. More broadly, literally any type ofsensing apparatus which produces an electrical output (whether formeasuring physiologic parameters or otherwise) may be utilizedconsistent with the invention.

The digital domain blood pressure data generated by the exemplarymeasurement system 1302 is fed to the interface circuit 1300, whichconditions the signal as previously described so as to allow seamlessdata communication with the selected patient monitor. As previouslydiscussed, the interface circuit 1300 is adapted to communicate datawith literally any type of patient monitor device (regardless ofmanufacturer), and hence the apparatus 1300 of FIG. 13 mayadvantageously be used to non-invasively measure blood pressure from asubject irrespective of the in situ patient monitoring equipmentavailable.

The apparatus of FIG. 13 is further adapted to be physically containedwithin a unitary or discrete device 1320 with associated electricalconnector 1322 such that the connector 1322 may be plugged directly intothe corresponding electrical receptacle within the in situ patientmonitoring device. Since the receptacle configuration generally variesfrom monitor to monitor, the connector 1322 may be configured to matewith an adapter 1324 which receives the connector 1322 of the device1320 in a first portion 1326, and mates with the associated receptacle1340 of the patient monitoring device 1342 via a second portion 1328. Inthe illustrated embodiment, the adapter 1324 comprises a female-maleplug (i.e., the connector 1322 of the NIBPM device 1320 is received in afemale connector 1330 in the first portion 1326, and the male connector1332 disposed on the second portion 1328 is received in thecorresponding patient monitor female receptacle 1340). Such patientmonitor female receptacle configuration is widely used for purposes ofreducing electrical shock hazard, since the electrical terminalscarrying excitation voltage to the sensing apparatus 1300 are shieldedfrom casual contact. It will be recognized, however, that otherarrangements may be employed in the adapter 1324 consistent with theinvention, including for example female/female, male/male, andmale/female (as differentiated from female/male in the embodiment ofFIG. 13). Furthermore, ganged or multiple adapter configurations may beemployed, such as in the case where it is desirable to have (i) two ormore NIBPM devices 1320 (or devices of mixed configuration) electricallycommunicating with a sole patient monitor 1342. Other configurations andvariations on the foregoing themes are possible, all such configurationsand variations falling within the scope of the claims appended hereto.

Furthermore, it will be readily apparent that the connector 1322 of theapparatus 1300 may be adapted to be received directly by or within thereceptacle 1340 of the selected monitor 1342. For example, if certainmedical/treatment facilities uniformly utilize only one type of patientmonitor, they may wish to procure NIBPM devices with connectors 1322which are adapted for direct receipt by the receptacle 1340 of that typeof patient monitor, thereby obviating the need for separate adapters(and any cost, safety, or electrical performance issues potentiallyrelating thereto.)

Referring now to FIG. 13 a, a second embodiment of the apparatus fornon-invasively assessing hemodynamic parameters associated with a livingsubject is described. In the embodiment of FIG. 12 a, the apparatus 1380is configured such that the interface circuit 800, 900, 1100 is disposedproximate to the patient monitor 1342, specifically as part of theadapter 1384 used to couple the connector 1322 to the patient monitorreceptacle 1340. The interface circuit components (including DAC,comparators, amplifiers, resistors, capacitors, etc.) are disposed on asmall form factor substrate 1386 such as a printed circuit board.Miniature circuit boards are well known in the microelectronic/surfacemount electronics arts, and accordingly are not described furtherherein. The interface circuit 800, 900, 1100 is then electricallydisposed in the circuit path between the NIBPM digital data output andpatient monitor receptacle 1340, such that (i) the digital data producedby the NIPBM is provided to the input terminals of, e.g., the DAC 830(see FIG. 8), and the ground, excitation (E_(d)), and referencepotentials necessary to operate the interface circuit are provided tothe appropriate points in the circuit 800, 900, 1100. The substrate 1386with components is physically contained in an over-molding 1390 whichencapsulates the assembly in a polymer/elastomeric material, therebyincreasing its durability and resistance to external effects such astemperature variation, moisture, dust, electromagnetic noise, andphysical trauma. The assembly is also optionally electromagneticallyshielded using, for example, a grounded tin-copper alloy metallic shieldelement of the type well known in the art, such shield being disposedwithin the over-molding 1390.

In yet another embodiment, shown as FIG. 14 herein, the apparatus ofFIG. 13 (and patient monitor) may be configured to include a wirelesslink 1402 between the apparatus 1400 and the patient monitor 1342, suchthat the interface circuitry 800, 900, 1100 is disposed proximate to thepatient monitor 1342. As illustrated in FIG. 14, the link 1402 comprisesa radio frequency (RF) communications system of the type well known inthe electrical arts. For example, in one exemplary variant, an RFtransceiver 1410 and modulator device 1412 are provided and adapted togenerally comply with the well known “Bluetooth™” wireless interfacestandard. The Bluetooth “3G” wireless technology allows users to makewireless and instant connections between various communication devices,such as mobile devices (e.g., cellular telephones, PDAs, notebookcomputers, local or remote patient monitoring stations, and the like)and desktop computers or other fixed devices. Since Bluetooth uses radiofrequency transmission, transfer of data is in real-time. The Bluetoothtopology supports both point-to-point and point-to-multipointconnections. Multiple ‘slave’ devices can be set to communicate with a‘master’ device. In this fashion, the NIBPM device 1406 of the presentinvention, when outfitted with a Bluetooth wireless suite, maycommunicate directly with other Bluetooth compliant mobile or fixeddevices including the patient monitor receiver 1427 disposed at themonitor 1342, or alternatively other Bluetooth-capable devices such as acellular telephone, PDA, notebook computer, or desktop computer.

The monitor receiver 1427 comprises a Bluetooth compatible transceiver1429 adapted to receive binary digital data in the form of radiated RFenergy from the corresponding transceiver 1410 on the NIBPM device, anddecode the binary data for input to the DAC 830 of the interfacecircuitry 800, 900, 1100. Hence, the operation of the wireless link 1402is effectively transparent to the interface circuit and patient monitor1342.

The monitor receiver unit 1427 is configured physically to mate with thepatient monitor receptacle 1340 (either directly or via an interposedadapter 1324, as previously described), such that the unit 1427 may besimply “plugged in” to the receptacle 1440 and remain free-standing. TheBluetooth transceiver 1429 of the unit 1427 and components of theinterface circuit 800, 900, 1100 (as well as other attendant electroniccomponents) are readily contained within the rectangular form factor(FIG. 14 a) of the receiver 1427, although it will be appreciated thatother forms may be utilized (such as a cylinder, sphere, square, etc.).Additionally, it will be recognized that for purposes of saving spacewithin the apparatus 1400, the signal processing andtransceiver/modulator components of the NIBPM device 1400 may beembodied in a fully integrated “system on a chip” (SoC) applicationspecific integrated circuit (ASIC) of the type generally known in thesemiconductor fabrication arts (not shown). The SoC ASIC incorporates,inter alia, a digital signal processor (DSP) core, embedded program anddata random access memories, RF transceiver circuitry, modulator,analog-to-digital converter (ADC), and analog interface circuitrynecessary to support sampling, conversion, processing, and transmissionof the blood pressure (or other) data to the receiver 1427. The SoCdevice design is generated using VHSIC Hardware Description language(VHDL) in conjunction with design and synthesis tools of the type wellknown in the art. A 0.18 micron MOS-based process is used to fabricatethe device of the illustrated embodiment, although other semiconductorfabrications processes including for example 0.35 micron or 0.1 micronmay be substituted, depending on the degree of integration required.

Alternatively, a number of different subjects undergoing blood pressuremonitoring/analysis using the NIBPM of the present invention (or othercomparable devices) may be monitored in real time at a centralizedlocation using a single monitor receiver 1427. Specifically, the monitorreceiver 1427 and transceiver 1429 are adapted to receive a plurality(currently seven, under prevailing Bluetooth architecture, although suchnumber may be increased or decreased) of signals from remote devices(e.g., NIBPMs), whereby the individual signals may be multiplexed oralternatively processed in parallel by the interface circuit 800, 900,1100 (with the addition of appropriate multiplexing or parallelprocessing hardware of the type well known in the electronic arts).Hence, a patient monitor 1342 configured to receive such multiplexed orparallel channel data may be used to monitor multiple subjects at once.

Bluetooth-compliant devices, inter alia, operate in the 2.4 GHz ISMband. The ISM band is dedicated to unlicensed users, including medicalfacilities, thereby advantageously allowing for unrestricted spectralaccess. Maximum radiated power levels from the transceiver 1410 of FIG.14 are in the mW range, thereby having no deleterious effect on thephysiology of the subject due to radiated electromagnetic energy. As iswell known in the wireless telecommunications art, radiated power fromthe antenna assembly (not shown) of the transceiver 1410 may also becontrolled and adjusted based on relative proximity of the transceiver1410, thereby further reducing electromagnetic whole body dose to thesubject.

The modulator 1412 of the illustrated embodiment uses one or morevariants of frequency shift keying, such as Gaussian Frequency ShiftKeying (GFSK) or Gaussian Minimum Shift keying (GMSK) of the type wellknown in the art to modulate data onto the carrier(s), although othertypes of modulation (such as phase modulation or amplitude modulation)may be used.

Spectral access of the device may be accomplished via frequency dividedmultiple access (FDMA), frequency hopping spread spectrum (FHSS), directsequence spread spectrum (DSSS, including code division multiple access)using a pseudo-noise spreading code, or even time division multipleaccess, depending on the needs of the user. For example, devicescomplying with IEEE Std. 802.11 may be substituted in the probe for theBluetooth transceiver/modulator arrangement previously described ifdesired. Literally any wireless interface capable of accommodating thebandwidth requirements of the system may be used. As yet anotherembodiment, an infrared device (e.g., Infrared Data Association “IrDA”)may be substituted or even used in conjunction with the aforementionedRF link 1402.

Method of Providing Treatment

Referring now to FIG. 15, a method of providing treatment to a subjectusing the aforementioned hemodynamic assessment and interface circuitapparatus is described in detail. It will be recognized that while thefollowing method of treatment is cast in terms of arterial bloodpressure monitoring for a living subject, parameters other than bloodpressure may be monitored using the methods of the invention, and othercourses of therapy or treatment provided.

As illustrated in FIG. 15, the first step 1502 of the method 1500comprises obtaining data from the subject using a sensing device such asthe NIPBM described previously herein. The data obtained from thesubject comprises hemodynamic data (e.g., pressure, velocity, kineticenergy) obtained from the subject, such as from the radial artery. Adigital domain signal is next generated in step 1504 based at least inpart on the data obtained in step 1502; blood pressure signal generationis described explicitly in the aforementioned co-pending U.S. patentapplications previously incorporated herein. Next, in step 1506, thedigital domain signal is conditioned using the interface circuit 800,900, 1100 to produce an analog signal. As previously described herein,the interface circuitry, inter alia, applies a transfer function to thedata after conversion from the digital domain to the analog domain, soas to substantially replicate the output which would be provided by adesired “target” device, in this case a passive resistor bridge circuitof the type well known in the art.

The conditioned analog signal is then provided to the patient monitoringdevice in step 1508, the latter producing a representation of thedesired parameter in real time. Such representation may comprise, forexample, calibrated displays the systolic, diastolic, and mean pressurewaveforms obtained from the subject, or alternatively otherrepresentations such as digital values of the systolic, diastolic, andmean pressures. Lastly, in step 1510, treatment is provided to theliving subject based on the parametric representation of step 1508. Thecaregiver may prescribe a course of treatment based on the displayedrepresentation, such as the administration pharmaceuticals, oradditional monitoring. Alternatively, such calibrated measurements maybe collected over an extended period of time and analyzed for long termtrends in the condition or response of the circulatory system of thesubject. As yet another alternative, the treatment may be automaticallyprovided based on the signals output to the patient monitor (orderivations thereof), for example such as sounding an alarm to drawattention to the rapidly falling mean blood pressure level of thepatient, or adjusting parameters associated with the NIBPM or othermonitoring devices being used on the subject.

It will be appreciated that while certain aspects of the invention havebeen described in terms of a specific sequence of steps of a method,these descriptions are only illustrative of the broader methods of theinvention, and may be modified as required by the particularapplication. Certain steps may be rendered unnecessary or optional undercertain circumstances. Additionally, certain steps or functionality maybe added to the disclosed embodiments, or the order of performance oftwo or more steps permuted. All such variations are considered to beencompassed within the invention disclosed and claimed herein.

While the above detailed description has shown, described, and pointedout novel features of the invention as applied to various embodiments,it will be understood that various omissions, substitutions, and changesin the form and details of the device or process illustrated may be madeby those skilled in the art without departing from the invention. Theforegoing description is of the best mode presently contemplated ofcarrying out the invention. This description is in no way meant to belimiting, but rather should be taken as illustrative of the generalprinciples of the invention. The scope of the invention should bedetermined with reference to the claims.

1.-13. (canceled)
 14. A method for simulating a time-variant outputsignal from a first device using a second device, comprising: providinga first device having at least one transfer function associatedtherewith; providing said second device, said second device beingadapted to sense at least one time-variant parameter and provide arepresentation based at least in part thereon; providing an excitationvoltage to said second device; generating a representation of saidtime-variant parameter using said second device; applying a transferfunction to said representation, said transfer function beingsubstantially similar to that for said first device; and generating atleast one output signal based at least in part on said representationand said transfer function, said at least one output signal beingsubstantially similar to that produced by said first device.
 15. Themethod of claim 14, wherein the act of sensing at least one time-variantparameter comprises sensing a pressure waveform from the blood vessel ofa living subject.
 16. The method of claim 15, wherein the act of sensingis performed non-invasively.
 17. The method of claim 15, wherein saidfirst device comprises a passive bridge element, and the act ofgenerating a representation comprises generating a binary digitalrepresentation.
 18. The method of claim 17, wherein the act of applyinga transfer function comprises: receiving said binary digitalrepresentation at a digital-to-analog converter (DAC); converting saidbinary digital representation to an analog representation; amplifyingsaid analog representation tp produce an amplified signal, saidamplified signal being provided to said DAC in a feedback loop andforming a floating current source; and producing a predeterminedrelationship between said amplified signal and an excitation signalapplied to said second device.
 19. The method of claim 16, furthercomprising buffering said excitation signal.
 20. The method of claim 14,further comprising detecting the electrical connection between saidsecond device and an externally applied excitation signal by detectingthe presence of a specified impedance value.
 21. The method of claim 14,further comprising detecting the electrical connection between saidsecond device and an externally applied excitation signal by detectingthe presence of a voltage at a predetermined terminal of the devicesupplying said excitation signal.
 22. A method of determining the statusof electrical connection between a monitoring device and a sensingdevice, wherein said monitoring device provides an excitation signal tosaid sensing device during operation, comprising: detecting the presenceof the excitation signal provided by said monitoring device; bufferingsaid excitation signal to produce a buffered excitation signal; andanalyzing said buffered excitation signal to identify variations thereinindicative of said status of electrical connection.
 23. The method ofclaim 22, wherein the act of analyzing comprises applying at least onetransfer function to said buffered excitation signal.
 24. The method ofclaim 23, wherein the act of applying comprises applying a windowcomparator transfer function.
 25. The method of claim 22, wherein theact of analyzing comprises specifying a time constant adapted tocompensate for zero-crossing waveforms or other short-duration voltagetransients.
 26. Apparatus, comprising: sensing apparatus adapted tosense at least one waveform, and generate a binary digitalrepresentation related thereto; and an interface circuit operativelycoupled to said sensing apparatus, said interface circuit being adaptedto applying a transfer function to said digital representation, andgenerate at least one output signal, said at least one output signalbased at least in part on said binary digital representation and saidtransfer function, said at least one output signal being substantiallysimilar to that produced by a passive bridge device.
 27. The apparatusof claim 26, wherein said sensing device comprises a non-invasive bloodpressure monitor.
 28. The apparatus of claim 27, wherein said binarydigital representation comprises data representative of blood pressureobtained from the blood vessel of a living subject.
 29. The apparatus ofclaim 28, wherein said binary digital representation is derived at leastin part by calculating time frequency distributions for said at leastone waveform.
 30. The apparatus of claim 28, wherein said binary digitalrepresentation is derived by: (i) measuring a first parameter from saidblood vessel; (ii) compressing said blood vessel; (iii) measuring asecond parameter from said blood vessel during said act of compressing;(iv) deriving a calibration function based at least in part on saidsecond parameter; and (v) calibrating the first parameter using saidcalibration function.
 31. The apparatus of claim 30, wherein said firstparameter comprises pressure within said blood vessel, and said secondparameter comprises blood flow kinetic energy.
 32. The apparatus ofclaim 26, further comprising a wireless communications interface, saidwireless interface being adapted for the communication of data betweensaid sensing apparatus and said interface circuit.
 33. The apparatus ofclaim 26, further comprising a wireless communications interface, saidwireless interface being adapted for the communication of data betweensaid apparatus and a monitoring device.
 34. The apparatus of claim 32,wherein said wireless interface utilizes radio frequency energy tocommunicate said data.
 35. The apparatus of claim 34, wherein said radiofrequency energy is disposed substantially within the ISM band.
 36. Theapparatus of claim 34, wherein said wireless interface comprises aspread spectrum device.
 37. Apparatus for monitoring a time-variantwaveform, comprising: a non-invasive blood pressure sensing device, saiddevice being configured to produce at least a binary digitalrepresentation related to said time variant waveform; interfacecircuitry in data communication with said sensing device, said interfacecircuitry being adapted to condition said at least binary digitalrepresentation according to at least one transfer function, and producean analog representation thereof; a monitoring device, in communicationwith at least said interface circuitry, adapted to utilize said analogsignal for the monitoring of said waveform.
 38. The apparatus of claim37, wherein said sensing device is adapted to sense at least one bloodpressure waveform, and said monitor is adapted to either analyze ordisplay information representative of said blood pressure waveform. 39.The apparatus of claim 37, wherein said at least one transfer functionsubstantially replicates that of a passive bridge transducer.
 40. Theapparatus of claim 38, wherein said at least one transfer functionsubstantially replicates that of a passive bridge transducer.
 41. Amethod of providing treatment to a living subject, comprising; obtainingdata from said subject using a sensing device; generating a first signalbased at least in part on said obtained data; conditioning said firstsignal using a conditioning circuit to produce a second signal;providing said second signal to a monitoring device, said monitoringdevice producing a representation of a desired parameter; and providingtreatment to said living subject based on said parametricrepresentation.
 42. The method of claim 41, wherein said act ofconditioning comprises applying a transfer function to said first signalto produce said second signal, said second signal substantiallysimulating that produced by a passive bridge device compatible with saidmonitoring device.
 43. The method of claim 42, wherein said act ofconditioning further comprises conditioning said first signal based atleast in part on an excitation signal provided by said monitoringdevice.
 44. The method of claim 42, further comprising: detecting anexcitation signal provided by said monitoring device, said excitationsignal being substantially unique to said monitoring device; andconditioning said first signal based at least in part on said excitationsignal; wherein said second signal is specifically configured foroperation with said monitoring device. 45.-52. (canceled)